First thing's first, in response to a few posters: the blue coins are not necessary to collect. Complaining about optional collectibles is invalid.
Now, for my reviews of both:
Sunshine began, from the outset, with a slow pace that would deteriorate the player until its unsavory finale. The levels required a good deal of precise controls to heights that not many other 3D action games go to. The level design was interesting in that you could maneuver faster and further if the player were good at the controls. The only big problem was, of course, repeating levels, and to the greatest extent in the Mario series. This was offset by some levels having some entirely different places to go to, but it still wasn't enough to make up for the lack of levels. Some stars are too easy to get, some annoyingly difficult, so it's obvious Nintendo EAD didn't playtest it enough. The bosses were lackluster and annoying, the final boss was also dumb, but the cave levels kind of made up for this at the same time. Some activities required of the player are downright chores. Cleaning up an island, at times, is just as dull as it sounds. But regardless of how much the player does so, Sunshine was and remains a mess.
Super Mario Galaxy took things way past the basic standards of 64 with the linear level design and straddled an easy difficulty which was once unheard of in the beloved series. The result was a fascinating loss of potential. While the basic level design was consistently creative, only so many of its 'innovative' ideas worked. The marvelously distorted gravity everywhere was hampered by a lack of creative ideas to go along with it. While graphically appealing, most levels that could have redesigned very similarly without it. In the long run, it was only a movement gimmick, even weaker than the FLUDD from Galaxy, only it didn't require anything of the player. While the game didn't seem very repetitive on the surface, underneath everything the basic level structure wasn't composed of too many different aspects. The enemies were stupidly easy to deal with from the beginning, and requiring the player to eliminate them at various times cut the pace down a lot. Not to mention all of the easy platforming. Pretty much anyone could've walk straight into the middle of the game and completed a level without much difficulty whatsoever, save for the messy controls required by the Wii Remote. Speaking of the controls, the shake-to-spin move was the worst move added to Mario's arsenal in the entire history of the entire platforming series. It was only justified as an addition by the developer with the addition of enemies that could only be killed by the spin. Basically, every single enemy in the game slowed the pace down. Never was it so utterly boring to waste time with enemies as threatening as domesticated dogs. The bosses were more interesting, only they were also too easy to kill. Bowser in particular was a creative battle the first time, but repeated two more times with only slightly faster attack speeds killed his galactic luster. As did the difficulty everywhere else. The music was as equally forgettable as the rest of the game, with only two or three memorable songs, and the rest already remixed to death.
Since 5.5>5, Sunshine is the better game. Or, should I say, the less worse.